feat(retry): per-gate retry budgets + progress-aware charging + hybrid salvage

Replaces the single shared per-stage retryCount (reset on TransitionExecuted,
shared across all post-stage gates) with a per-gate budget. Motivated by run
2e468f9f, where a promising freestyle run was terminally FAILED because the
contract gate burned all 3 shared retries on one trivial miss before the
semantic-review gate ever ran.

- StageExecutionResult.Failure gains a `gate` id; each gate tags its failures.
- OrchestrationState gains gateRetryBudgets / gateFailureFingerprints /
  gateSalvageUsed (all rebuilt from events, reset per-stage on TransitionExecuted).
- FailureFingerprint normalizes+hashes the failure reason; RetryCoordinator.decide
  charges a gate's budget only when the fingerprint is unchanged (no progress),
  so a moving run is never penalised. Returns Retry | Exhausted.
- Hybrid exhaustion: deterministic gates fail; the review gate consults a
  SalvageJudge (LLM, or a deterministic allow-one-reset fallback), recorded as
  RetrySalvageDecidedEvent (invariant #9). CONTINUE resets the gate budget once;
  a second exhaustion is terminal.
- Review gate is now the sole authority on review-retry termination; the old
  REVIEW_BLOCK_RETRY_CAP is repurposed as a high absolute backstop only.
- ServerModule escaped-exception path now records a truthful terminal
  WorkflowFailed (real stage/reason/retryExhausted) via recordUnhandledFailure.

Tests: DefaultRetryCoordinatorTest (fingerprint charging) +
GateRetryBudgetExhaustionTest (deterministic exhaust / review CONTINUE-then-
terminal / review FAIL / null-judge fallback) + reducer coverage.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/DefaultOrchestrationReducer.kt

@@ -5,6 +5,8 @@ import com.correx.core.events.events.OrchestrationPausedEvent
 import com.correx.core.events.events.OrchestrationResumedEvent
 import com.correx.core.events.events.RefinementIterationEvent
 import com.correx.core.events.events.RetryAttemptedEvent
+import com.correx.core.events.events.RetrySalvageDecidedEvent
+import com.correx.core.events.events.SalvageDecision
 import com.correx.core.events.events.StoredEvent
 import com.correx.core.events.events.TransitionExecutedEvent
 import com.correx.core.events.events.WorkflowCompletedEvent
@@ -59,14 +61,35 @@ class DefaultOrchestrationReducer : OrchestrationReducer {
         // turning the next stage's first retryable failure into a terminal WorkflowFailed (live repro:
         // implement burned 3/3, then review's retryable JSON-decode error got 0 retries). Back-edge
         // loops stay bounded by the separate refinementIterations counter, so this can't loop forever.
-        is TransitionExecutedEvent -> state.copy(currentStageId = p.to, retryCount = 0)
+        is TransitionExecutedEvent -> state.copy(
+            currentStageId = p.to,
+            retryCount = 0,
+            gateRetryBudgets = emptyMap(),
+            gateFailureFingerprints = emptyMap(),
+            gateSalvageUsed = emptySet(),
+        )
 
         is RetryAttemptedEvent -> state.copy(
             status = OrchestrationStatus.RUNNING,
             retryCount = p.attemptNumber,
             failureReason = null,
+            gateRetryBudgets = if (p.charged) {
+                state.gateRetryBudgets + (p.gate to ((state.gateRetryBudgets[p.gate] ?: 0) + 1))
+            } else {
+                state.gateRetryBudgets
+            },
+            gateFailureFingerprints = state.gateFailureFingerprints + (p.gate to p.fingerprint),
         )
 
+        is RetrySalvageDecidedEvent -> if (p.decision == SalvageDecision.CONTINUE) {
+            state.copy(
+                gateRetryBudgets = state.gateRetryBudgets + (p.gate to 0),
+                gateSalvageUsed = state.gateSalvageUsed + p.gate,
+            )
+        } else {
+            state
+        }
+
         is RefinementIterationEvent -> state.copy(
             refinementIterations = state.refinementIterations + (p.cycleKey to p.iteration),
         )

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/DefaultSessionOrchestrator.kt

@@ -17,6 +17,8 @@ import com.correx.core.events.events.ClarificationAnsweredEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
 import com.correx.core.events.events.OrchestrationResumedEvent
 import com.correx.core.events.events.RefinementIterationEvent
+import com.correx.core.events.events.RetrySalvageDecidedEvent
+import com.correx.core.events.events.SalvageDecision
 import com.correx.core.events.events.StoredEvent
 import com.correx.core.events.events.TransitionExecutedEvent
 import com.correx.core.events.orchestration.OrchestrationState
@@ -32,6 +34,7 @@ import com.correx.core.kernel.orchestration.subagent.InSessionSubagentRunner
 import com.correx.core.kernel.orchestration.subagent.SubagentRunRequest
 import com.correx.core.kernel.orchestration.subagent.SubagentRunner
 import com.correx.core.kernel.retry.RetryCoordinator
+import com.correx.core.kernel.retry.RetryDecision
 import com.correx.core.sessions.Session
 import com.correx.core.transitions.execution.StageExecutionResult
 import com.correx.core.transitions.graph.WorkflowGraph
@@ -68,6 +71,11 @@ class DefaultSessionOrchestrator(
     override val cancellations: ConcurrentHashMap<SessionId, AtomicBoolean> =
         ConcurrentHashMap<SessionId, AtomicBoolean>()
 
+    // Hybrid-exhaustion salvage judge (T6/T7): consulted only when the "review" gate exhausts its
+    // per-gate retry budget. Null = deterministic allow-one-reset-then-fail fallback (see
+    // decideGateExhaustion) so the feature degrades safely without inference.
+    private val salvageJudge: SalvageJudge? = engines.salvageJudge
+
     override val subagentRunner: SubagentRunner = InSessionSubagentRunner(
         executeStage = { sid, stg, graph, session, cfg ->
             executeStage(sid, stg, graph, session, cfg, effectivesFor(cfg))
@@ -442,29 +450,64 @@ class DefaultSessionOrchestrator(
                         return StepResult.Continue(ctx.copy(stageCount = ctx.stageCount + 1))
                     }
                     val refreshedState = orchestrationRepository.getState(ctx.sessionId)
-                    val shouldRetry = retryCoordinator.shouldRetry(
+                    val gateDecision = retryCoordinator.decide(
                         sessionId = ctx.sessionId,
                         stageId = stageId,
-                        currentAttempt = refreshedState.retryCount,
-                        policy = ctx.config.retryPolicy,
+                        gate = result.gate,
                         failureReason = result.reason,
+                        state = refreshedState,
+                        policy = ctx.config.retryPolicy,
                     )
-                    if (!shouldRetry) {
-                        return StepResult.Terminal(
-                            failWorkflow(
-                                ctx.sessionId,
-                                stageId,
-                                result.reason,
-                                retryExhausted = true,
-                            ),
-                        )
+                    when (gateDecision) {
+                        RetryDecision.Retry -> Unit // retry — loop and re-execute
+                        RetryDecision.Exhausted -> {
+                            val terminal = decideGateExhaustion(
+                                ctx.sessionId, stageId, result.gate, result.reason, refreshedState,
+                            )
+                            if (terminal != null) return StepResult.Terminal(terminal)
+                            // review-gate salvage said CONTINUE — budget was reset by the reducer;
+                            // loop and re-execute the stage with a fresh budget.
+                        }
                     }
-                    // retry — loop and re-execute
                 }
             }
         }
     }
 
+    /**
+     * Hybrid exhaustion (design 2026-07-06-per-gate-retry-budgets.md §3/T6): a deterministic gate
+     * exhausting its budget fails the stage terminally, unchanged from today's behaviour (just
+     * per-gate now). The "review" gate instead gets one salvageability judgement — CONTINUE resets
+     * its budget (via the reducer folding [RetrySalvageDecidedEvent]) and the caller loops to retry;
+     * FAIL is terminal. A gate that already spent its one salvage reset (state.gateSalvageUsed) fails
+     * immediately on a second exhaustion, regardless of what the judge would say.
+     *
+     * Returns the terminal [WorkflowResult] to fail with, or null to retry (loop and re-execute).
+     */
+    private suspend fun decideGateExhaustion(
+        sessionId: SessionId,
+        stageId: StageId,
+        gate: String,
+        reason: String,
+        state: OrchestrationState,
+    ): WorkflowResult? {
+        if (gate != "review" || state.gateSalvageUsed.contains(gate)) {
+            return failWorkflow(sessionId, stageId, reason, retryExhausted = true)
+        }
+        // No judge wired: degrade safely with a deterministic allow-one-reset-then-fail policy —
+        // the gateSalvageUsed check above already ensures this fires at most once per gate.
+        val judgment = salvageJudge?.judge(sessionId, stageId, reason)
+            ?: SalvageJudgment(
+                SalvageDecision.CONTINUE,
+                "no salvage judge wired — deterministic one-time reset",
+            )
+        emit(sessionId, RetrySalvageDecidedEvent(sessionId, stageId, gate, judgment.decision, judgment.rationale))
+        return when (judgment.decision) {
+            SalvageDecision.CONTINUE -> null
+            SalvageDecision.FAIL -> failWorkflow(sessionId, stageId, reason, retryExhausted = true)
+        }
+    }
+
     private fun ExecutionContext.enrich() = EnrichedExecutionContext(
         graph, sessionId, stageCount, currentStageId, config,
         session = repositories.sessionRepository.getSession(sessionId),

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/OrchestratorEngines.kt

@@ -43,4 +43,8 @@ data class OrchestratorEngines(
     // Gate 3). Null = no semantic review; the funnel then rests on the deterministic gates only. A
     // stage opts in via StageConfig.semanticReview.
     val semanticReviewer: SemanticReviewer? = null,
+    // Hybrid-exhaustion salvage judge (design 2026-07-06-per-gate-retry-budgets.md §3/T7), consulted
+    // only when the "review" gate exhausts its retry budget. Null = no LLM judge wired; the
+    // orchestrator then falls back to a deterministic allow-one-reset-then-fail policy.
+    val salvageJudge: SalvageJudge? = null,
 )

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SalvageJudge.kt

@@ -0,0 +1,24 @@
+package com.correx.core.kernel.orchestration
+
+import com.correx.core.events.events.SalvageDecision
+import com.correx.core.events.types.SessionId
+import com.correx.core.events.types.StageId
+
+/** Result of a [SalvageJudge.judge] call. */
+data class SalvageJudgment(val decision: SalvageDecision, val rationale: String)
+
+/**
+ * Seam for the hybrid-exhaustion salvage judgement (design 2026-07-06-per-gate-retry-budgets.md
+ * §3/T7): consulted when the "review" gate exhausts its per-gate retry budget. Like the other gate
+ * seams ([SemanticReviewer], [ContractAssertionEvaluator]), the inference-touching implementation is
+ * injected so the deterministic core never runs inference itself.
+ *
+ * The judgement is nondeterministic (LLM-backed), so invariant #9 requires the caller to record the
+ * outcome as a [com.correx.core.events.events.RetrySalvageDecidedEvent] — replay reads that event
+ * back and never re-invokes this judge. When no judge is wired (`null` in [OrchestratorEngines]),
+ * the orchestrator falls back to a deterministic allow-one-reset-then-fail policy so the feature
+ * degrades safely without inference (see `DefaultSessionOrchestrator.decideSalvage`).
+ */
+fun interface SalvageJudge {
+    suspend fun judge(sessionId: SessionId, stageId: StageId, reason: String): SalvageJudgment
+}

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -223,12 +223,16 @@ private const val MAX_CLARIFICATION_ROUNDS = 3
 // Static-analysis output caps: the tail retained in the recorded event vs. the (larger) slice fed
 // back to the model so it can fix the failure. Tails, because the error summary sits at the end.
 private const val CONTRACT_EVIDENCE_CAP = 400
-// Gate 3 semantic review: a FAIL blocks only on a correctness finding at/above this confidence, and
-// only until this many prior blocks have occurred for the stage — after which findings surface
-// without blocking, so a stuck reviewer can never trap a stage. Objective text fed to the reviewer
-// is capped to keep the review prompt bounded.
+// Gate 3 semantic review: a FAIL blocks only on a correctness finding at/above this confidence.
+// Per-gate retry budgeting + hybrid salvage (design 2026-07-06-per-gate-retry-budgets.md) is now the
+// authority on when review retries stop — the gate simply reports the block and the RetryCoordinator
+// charges/exhausts the "review" budget, then the salvage judge decides continue-vs-fail. This cap
+// remains only as an absolute backstop against a pathological run whose review findings *change*
+// every attempt (so the budget never charges, i.e. the run looks like perpetual "progress"): after
+// this many blocks, findings surface without blocking so a stuck reviewer can never trap a stage
+// forever. Set well above any legitimate budget+salvage sequence. Objective text is capped too.
 private const val REVIEW_BLOCK_MIN_CONFIDENCE = 0.7
-private const val REVIEW_BLOCK_RETRY_CAP = 2
+private const val REVIEW_BLOCK_RETRY_CAP = 20
 private const val REVIEW_OBJECTIVE_CAP = 4_000
 private const val STATIC_ANALYSIS_SUMMARY_CAP = 2_000
 private const val STATIC_ANALYSIS_FEEDBACK_CAP = 6_000
@@ -1929,6 +1933,7 @@ abstract class SessionOrchestrator(
             return StageExecutionResult.Failure(
                 "stage ${stageId.value} declared no artifacts and ran no tools",
                 retryable = true,
+                gate = "produces",
             )
         }
         val producedIds = stageConfig.produces.map { it.name }.toSet()
@@ -1945,6 +1950,7 @@ abstract class SessionOrchestrator(
         return StageExecutionResult.Failure(
             "stage ${stageId.value} did not produce declared artifacts: $missingIds",
             retryable = true,
+            gate = "produces",
         )
     }
 
@@ -2015,6 +2021,7 @@ abstract class SessionOrchestrator(
                 "stage ${stageId.value} references paths that do not exist in the workspace: " +
                     "${missing.joinToString(", ")}. Reference only files you have read.",
                 retryable = true,
+                gate = "brief_grounding",
             )
         }
     }
@@ -2077,6 +2084,7 @@ abstract class SessionOrchestrator(
                     "files not in the brief: [${div.hallucinatedFiles.joinToString(", ")}]. " +
                     "Restate the brief exactly; do not drop requirements or invent files.",
                 retryable = true,
+                gate = "brief_echo",
             )
         } else {
             StageExecutionResult.Success(emptyList())
@@ -2174,6 +2182,7 @@ abstract class SessionOrchestrator(
         return StageExecutionResult.Failure(
             "stage ${stageId.value} did not satisfy its file contract. Fix these before review:\n$detail",
             retryable = true,
+            gate = "contract",
         )
     }
 
@@ -2209,6 +2218,7 @@ abstract class SessionOrchestrator(
             "stage ${stageId.value} produced an execution_plan that does not compile. " +
                 "Fix the plan before it can be locked:\n$error",
             retryable = true,
+            gate = "plan_compile",
         )
     }
 
@@ -2283,6 +2293,7 @@ abstract class SessionOrchestrator(
         return StageExecutionResult.Failure(
             "stage ${stageId.value} did not pass static analysis. Fix these before review:\n\n$detail",
             retryable = true,
+            gate = "static_analysis",
         )
     }
 
@@ -2330,6 +2341,7 @@ abstract class SessionOrchestrator(
                 "Fix these before proceeding:\n\n$ $command  (exit ${run.exitCode})\n" +
                 run.output.takeLast(STATIC_ANALYSIS_FEEDBACK_CAP),
             retryable = true,
+            gate = "execution",
         )
     }
 
@@ -2402,6 +2414,7 @@ abstract class SessionOrchestrator(
         return StageExecutionResult.Failure(
             "stage ${stageId.value} failed semantic review — fix these correctness issues:\n$detail",
             retryable = true,
+            gate = "review",
         )
     }
 
@@ -2560,6 +2573,7 @@ abstract class SessionOrchestrator(
             if (isCancelled(sessionId)) return InferenceResult.Cancelled
 
             val responseArtifactId = artifactStore.put(response.text.toByteArray())
+            val reasoningArtifactId = response.reasoning?.let { artifactStore.put(it.toByteArray()) }
             emit(
                 sessionId,
                 InferenceCompletedEvent(
@@ -2570,6 +2584,7 @@ abstract class SessionOrchestrator(
                     tokensUsed = response.tokensUsed,
                     latencyMs = response.latencyMs,
                     responseArtifactId = responseArtifactId,
+                    reasoningArtifactId = reasoningArtifactId,
                 ),
             )
 
@@ -2687,8 +2702,35 @@ abstract class SessionOrchestrator(
             "[Orchestrator] FAILED session={} stage={} reason={} retryExhausted={}",
             sessionId.value, stageId.value, reason, retryExhausted,
         )
-        correlateCritiqueOutcomes(sessionId)
-        emit(sessionId, WorkflowFailedEvent(sessionId, stageId, reason, retryExhausted))
+        // failWorkflow IS the clean-termination path: whatever goes wrong while recording it
+        // (event-store hiccup, correlation lookup, or any other transient failure) must never be
+        // allowed to escape as a raw exception — that would blow past this function's own
+        // WorkflowFailedEvent (correct stageId/reason/retryExhausted) and land in the server's
+        // top-level catch-all, which fabricates its own event with the wrong stage (graph.start)
+        // and a garbage reason (the escaping throwable's message/toString). Record what we can and
+        // still return a clean Failed result either way.
+        // Critique correlation is best-effort bookkeeping; if it throws we log and press on, because
+        // the terminal WorkflowFailedEvent below MUST still be recorded — skipping it would leave the
+        // event log with no terminal marker for the session (breaks replay-completeness, invariants
+        // #1/#8) and let the server catch-all fabricate a wrong one.
+        runCatching { correlateCritiqueOutcomes(sessionId) }.onFailure { e ->
+            log.error(
+                "[Orchestrator] failWorkflow: critique correlation failed session={} stage={}",
+                sessionId.value, stageId.value, e,
+            )
+        }
+        // Last-ditch: the one unrecoverable case. If emitting the terminal event itself throws
+        // (e.g. a serialization edge), we cannot record the failure at all — log it loudly with the
+        // full throwable so it is never silent, then still return a clean Failed result.
+        runCatching {
+            emit(sessionId, WorkflowFailedEvent(sessionId, stageId, reason, retryExhausted))
+        }.onFailure { e ->
+            log.error(
+                "[Orchestrator] failWorkflow: FAILED to record terminal WorkflowFailedEvent — event " +
+                    "log has no terminal marker for session={} stage={}",
+                sessionId.value, stageId.value, e,
+            )
+        }
         cancellations.remove(sessionId)
         return WorkflowResult.Failed(sessionId, reason, retryExhausted)
     }
@@ -2961,6 +3003,7 @@ abstract class SessionOrchestrator(
 private suspend fun computeToolPreview(toolName: String, parameters: Map<String, Any>): String? {
     if (toolName == "shell") return shellCommandPreview(parameters)
     if (toolName == "task_decompose") return renderDecomposePreview(parameters)
+    if (toolName == "file_edit") return computeFileEditPreview(parameters)
     if (toolName != "file_write") return null
     val path = parameters["path"] as? String ?: return null
     // file_write no longer carries an `operation` param (delete was split into file_delete), so the
@@ -2968,13 +3011,39 @@ private suspend fun computeToolPreview(toolName: String, parameters: Map<String,
     // silently bailed to the raw-JSON args fallback for every file_write.
     val proposedContent = parameters["content"] as? String ?: return null
 
-    val existingContent = withContext(Dispatchers.IO) {
-        runCatching {
-            val filePath = java.nio.file.Paths.get(path)
-            if (java.nio.file.Files.exists(filePath)) {
-                java.nio.file.Files.readString(filePath)
-            } else null
-        }.getOrNull()
+    val existingContent = readFileIfExists(path)
+    return buildDiffString(path, existingContent, proposedContent)
+}
+
+private suspend fun readFileIfExists(path: String): String? = withContext(Dispatchers.IO) {
+    runCatching {
+        val filePath = java.nio.file.Paths.get(path)
+        if (java.nio.file.Files.exists(filePath)) {
+            java.nio.file.Files.readString(filePath)
+        } else null
+    }.getOrNull()
+}
+
+/**
+ * Preview for `file_edit`'s "replace" and "append" operations: reads the existing file and
+ * simulates the edit locally to produce a diff, mirroring `file_write`'s preview. "patch" is left
+ * to the raw-JSON-args fallback — simulating `patch -p1` application here isn't worth the risk of
+ * the preview silently disagreeing with what the tool actually does.
+ */
+private suspend fun computeFileEditPreview(parameters: Map<String, Any>): String? {
+    val path = parameters["path"] as? String ?: return null
+    val operation = parameters["operation"] as? String ?: return null
+    val existingContent = readFileIfExists(path) ?: return null
+
+    val proposedContent = when (operation) {
+        "append" -> existingContent + (parameters["content"] as? String ?: return null)
+        "replace" -> {
+            val target = parameters["target"] as? String ?: return null
+            val replacement = parameters["replacement"] as? String ?: return null
+            if (!existingContent.contains(target)) return null
+            existingContent.replaceFirst(target, replacement)
+        }
+        else -> return null
     }
 
     return buildDiffString(path, existingContent, proposedContent)
@@ -3003,25 +3072,136 @@ private fun shellQuoteToken(token: String): String {
     return if (needsQuote) "'" + token.replace("'", "'\\''") + "'" else token
 }
 
+private enum class DiffOpKind { EQUAL, DELETE, INSERT }
+private data class DiffOp(val kind: DiffOpKind, val line: String)
+
 /**
- * Build a unified-diff-style string for a full file replacement.
+ * Build a unified-diff string for a full file replacement, using a line-level LCS diff
+ * so a small edit to a large file shows as a tight hunk rather than a full-file replacement.
  * When the file doesn't exist yet (new file), only `+` lines are shown.
- * Uses a simple full-file diff: all old lines as `-`, all new lines as `+`.
  */
 private fun buildDiffString(path: String, existingContent: String?, proposedContent: String): String {
     val existingLines = existingContent?.lines() ?: emptyList()
     val proposedLines = proposedContent.lines()
     if (existingLines == proposedLines) return "  (no change)"
 
+    val ops = diffLines(existingLines, proposedLines)
     return buildString {
         appendLine("--- a/$path")
         appendLine("+++ b/$path")
-        appendLine("@@ -1,${existingLines.size.coerceAtLeast(1)} +1,${proposedLines.size.coerceAtLeast(1)} @@")
-        existingLines.forEach { appendLine("-$it") }
-        proposedLines.forEach { appendLine("+$it") }
+        append(renderHunks(ops))
     }.trimEnd()
 }
 
+/** Line-level LCS diff producing an edit script of EQUAL/DELETE/INSERT ops. */
+private fun diffLines(oldLines: List<String>, newLines: List<String>): List<DiffOp> {
+    val n = oldLines.size
+    val m = newLines.size
+    val lcs = Array(n + 1) { IntArray(m + 1) }
+    for (i in n - 1 downTo 0) {
+        for (j in m - 1 downTo 0) {
+            lcs[i][j] = if (oldLines[i] == newLines[j]) {
+                lcs[i + 1][j + 1] + 1
+            } else {
+                maxOf(lcs[i + 1][j], lcs[i][j + 1])
+            }
+        }
+    }
+    val ops = mutableListOf<DiffOp>()
+    var i = 0
+    var j = 0
+    while (i < n && j < m) {
+        when {
+            oldLines[i] == newLines[j] -> {
+                ops += DiffOp(DiffOpKind.EQUAL, oldLines[i])
+                i++
+                j++
+            }
+            lcs[i + 1][j] >= lcs[i][j + 1] -> {
+                ops += DiffOp(DiffOpKind.DELETE, oldLines[i])
+                i++
+            }
+            else -> {
+                ops += DiffOp(DiffOpKind.INSERT, newLines[j])
+                j++
+            }
+        }
+    }
+    while (i < n) {
+        ops += DiffOp(DiffOpKind.DELETE, oldLines[i])
+        i++
+    }
+    while (j < m) {
+        ops += DiffOp(DiffOpKind.INSERT, newLines[j])
+        j++
+    }
+    return ops
+}
+
+/** Groups an edit script into unified-diff hunks with [context] lines of surrounding EQUAL context. */
+private fun renderHunks(ops: List<DiffOp>, context: Int = 3): String {
+    val changedIndices = ops.indices.filter { ops[it].kind != DiffOpKind.EQUAL }
+    if (changedIndices.isEmpty()) return ""
+
+    data class Hunk(val start: Int, val end: Int)
+    val hunks = mutableListOf<Hunk>()
+    var hunkStart = changedIndices.first()
+    var hunkEnd = changedIndices.first()
+    for (idx in changedIndices.drop(1)) {
+        if (idx - hunkEnd <= context * 2) {
+            hunkEnd = idx
+        } else {
+            hunks += Hunk(hunkStart, hunkEnd)
+            hunkStart = idx
+            hunkEnd = idx
+        }
+    }
+    hunks += Hunk(hunkStart, hunkEnd)
+
+    var oldLine = 1
+    var newLine = 1
+    var opIdx = 0
+    return buildString {
+        for (hunk in hunks) {
+            val from = maxOf(0, hunk.start - context)
+            val to = minOf(ops.size - 1, hunk.end + context)
+            while (opIdx < from) {
+                if (ops[opIdx].kind != DiffOpKind.INSERT) oldLine++
+                if (ops[opIdx].kind != DiffOpKind.DELETE) newLine++
+                opIdx++
+            }
+            val oldStart = oldLine
+            val newStart = newLine
+            var oldCount = 0
+            var newCount = 0
+            val lines = StringBuilder()
+            for (k in from..to) {
+                val op = ops[k]
+                when (op.kind) {
+                    DiffOpKind.EQUAL -> {
+                        lines.append(' ').append(op.line).append('\n')
+                        oldCount++
+                        newCount++
+                    }
+                    DiffOpKind.DELETE -> {
+                        lines.append('-').append(op.line).append('\n')
+                        oldCount++
+                    }
+                    DiffOpKind.INSERT -> {
+                        lines.append('+').append(op.line).append('\n')
+                        newCount++
+                    }
+                }
+            }
+            appendLine("@@ -$oldStart,${oldCount.coerceAtLeast(1)} +$newStart,${newCount.coerceAtLeast(1)} @@")
+            append(lines)
+            oldLine = oldStart + oldCount
+            newLine = newStart + newCount
+            opIdx = to + 1
+        }
+    }
+}
+
 /**
  * The candidate in [paths] that shares [guessed]'s filename and the most trailing path segments,
  * or null when nothing shares the filename. Lets a wrong package/dir prefix

core/kernel/src/main/kotlin/com/correx/core/kernel/retry/DefaultRetryCoordinator.kt

@@ -4,6 +4,7 @@ import com.correx.core.events.events.EventMetadata
 import com.correx.core.events.events.NewEvent
 import com.correx.core.events.events.RetryAttemptedEvent
 import com.correx.core.events.execution.RetryPolicy
+import com.correx.core.events.orchestration.OrchestrationState
 import com.correx.core.events.stores.EventStore
 import com.correx.core.events.types.EventId
 import com.correx.core.events.types.SessionId
@@ -16,6 +17,53 @@ class DefaultRetryCoordinator(
     private val eventStore: EventStore,
 ) : RetryCoordinator {
 
+    override suspend fun decide(
+        sessionId: SessionId,
+        stageId: StageId,
+        gate: String,
+        failureReason: String,
+        state: OrchestrationState,
+        policy: RetryPolicy,
+    ): RetryDecision {
+        val fingerprint = FailureFingerprint.of(failureReason)
+        val prevFingerprint = state.gateFailureFingerprints[gate]
+        val progressed = prevFingerprint != fingerprint
+        val charged = !progressed
+        val currentCount = state.gateRetryBudgets[gate] ?: 0
+        val newCount = if (charged) currentCount + 1 else currentCount
+        val maxAttempts = policy.maxAttemptsFor(gate)
+        if (charged && newCount > maxAttempts) {
+            return RetryDecision.Exhausted
+        }
+
+        eventStore.append(
+            NewEvent(
+                metadata = EventMetadata(
+                    eventId = EventId(UUID.randomUUID().toString()),
+                    sessionId = sessionId,
+                    timestamp = Clock.System.now(),
+                    schemaVersion = 1,
+                    causationId = null,
+                    correlationId = null,
+                ),
+                payload = RetryAttemptedEvent(
+                    sessionId = sessionId,
+                    stageId = stageId,
+                    attemptNumber = newCount,
+                    maxAttempts = maxAttempts,
+                    failureReason = failureReason,
+                    gate = gate,
+                    fingerprint = fingerprint,
+                    charged = charged,
+                ),
+            ),
+        )
+        if (policy.backoffMs > 0L) {
+            delay(policy.backoffMs)
+        }
+        return RetryDecision.Retry
+    }
+
     override suspend fun shouldRetry(
         sessionId: SessionId,
         stageId: StageId,

core/kernel/src/main/kotlin/com/correx/core/kernel/retry/FailureFingerprint.kt

@@ -0,0 +1,21 @@
+package com.correx.core.kernel.retry
+
+// Collapses whitespace runs and digit runs so line numbers / byte counts / timestamps embedded in a
+// gate's failure reason don't make two otherwise-identical failures look like "progress". Gate
+// reasons already list the failing assertion/finding ids verbatim (see runContractGate,
+// runStaticAnalysis, runReviewGate in SessionOrchestrator), so this normalisation is sufficient to
+// treat "same root cause" retries as no-progress without needing per-gate parsing.
+private val VOLATILE_RUN = Regex("""[\s\d]+""")
+
+/**
+ * Stable fingerprint of a [StageExecutionResult.Failure] reason (design
+ * 2026-07-06-per-gate-retry-budgets.md §5). Two failures with the same fingerprint are treated as
+ * "no progress" for per-gate retry budgeting; a changed fingerprint means the run is moving and the
+ * retry is free.
+ */
+object FailureFingerprint {
+    fun of(reason: String): String {
+        val normalized = reason.trim().lowercase().replace(VOLATILE_RUN, " ").trim()
+        return Integer.toHexString(normalized.hashCode())
+    }
+}

core/kernel/src/main/kotlin/com/correx/core/kernel/retry/RetryCoordinator.kt

@@ -1,10 +1,21 @@
 package com.correx.core.kernel.retry
 
 import com.correx.core.events.execution.RetryPolicy
+import com.correx.core.events.orchestration.OrchestrationState
 import com.correx.core.events.types.SessionId
 import com.correx.core.events.types.StageId
 
+/** Outcome of [RetryCoordinator.decide] for a single gate's failure. */
+sealed interface RetryDecision {
+    data object Retry : RetryDecision
+    data object Exhausted : RetryDecision
+}
+
 interface RetryCoordinator {
+    /**
+     * Legacy shared-budget entry point, kept for the non-gate transition-failure paths
+     * (Stay/NoMatch in DefaultSessionOrchestrator.step) that predate per-gate budgeting.
+     */
     suspend fun shouldRetry(
         sessionId: SessionId,
         stageId: StageId,
@@ -12,4 +23,23 @@ interface RetryCoordinator {
         policy: RetryPolicy,
         failureReason: String,
     ): Boolean
+
+    /**
+     * Per-gate, progress-aware retry decision (design 2026-07-06-per-gate-retry-budgets.md §5).
+     * [failureReason] is fingerprinted internally (see [FailureFingerprint]) and charges [gate]'s
+     * budget only when the fingerprint is unchanged from the last attempt recorded in [state] for
+     * that gate (no progress); a changed fingerprint is free. On a charging decision that would
+     * exceed the gate's budget, returns [RetryDecision.Exhausted] without emitting an event — the
+     * caller (hybrid exhaustion, T6) decides what happens next. Otherwise emits
+     * [com.correx.core.events.events.RetryAttemptedEvent] (with the fingerprint and the full
+     * [failureReason] both recorded), applies backoff, and returns [RetryDecision.Retry].
+     */
+    suspend fun decide(
+        sessionId: SessionId,
+        stageId: StageId,
+        gate: String,
+        failureReason: String,
+        state: OrchestrationState,
+        policy: RetryPolicy,
+    ): RetryDecision
 }