fix(anthropic): route input_json deltas by content-block index

[mcavage-docker]

Summary

Parallel tool_use blocks streamed from Anthropic share a single streamAdapter.toolID field that gets overwritten on every ContentBlockStartEvent. Subsequent InputJSONDelta events for the first block carry the second block's tool ID, and the runtime accumulator (keyed by ToolCall.ID in pkg/runtime/streaming.go) concatenates both calls' argument fragments into one buffer. The first tool's args end up empty; the second tool's args contain interleaved fragments that fail to parse, surfacing as:

invalid character 's' looking for beginning of value
invalid character '-' after object key:value pair

(the exact character depends on where the chunk boundaries fall.)

This fix tracks tool IDs per content-block index via toolIDByBlock map[int64]string and routes InputJSONDelta events using eventVariant.Index. The same fix is applied to betaStreamAdapter, which shared the bug.

How it manifests in production

Observed against a local GM agent team (cagent v1.57.0) running Claude Opus with ~18 parallel MCP tool calls in a single turn. Five calls failed with the JSON errors above before reaching the MCP gateway; the other thirteen succeeded. Retrying the same calls in isolation always succeeded — which made it look like a transient parser corruption issue, but the actual condition is deterministic: it only fires when the provider interleaves input_json_delta events across blocks, which is more likely under heavier fan-out.

The bug is present on current main (verified at HEAD 03386b31). Bug was never reported via gh search for "parallel tool", "invalid character", or "tool call arguments" before this PR.

Test plan

pkg/model/provider/anthropic/parallel_toolcall_test.go (added in this PR) feeds a fake SSE decoder a sequence of two parallel tool_use blocks with interleaved input_json_delta events, mirroring what Anthropic emits for parallel tool calls. Covers both streamAdapter and betaStreamAdapter.

Without the fix:

toolu_AAA args=\"\"
toolu_BBB args=\`{\"refresh_id\":\"abc-{\"category\":\"tool_failure\",def\"}\"summary\":\"x\"}\`
JSON parse error: invalid character 'c' after object key:value pair

With the fix:

toolu_AAA args=\`{\"refresh_id\":\"abc-def\"}\`
toolu_BBB args=\`{\"category\":\"tool_failure\",\"summary\":\"x\"}\`

• New regression tests added (`TestParallelToolCallIDsAreNotCrossWired`, `TestBetaParallelToolCallIDsAreNotCrossWired`)
• Both new tests fail on `main` without the patch
• Both new tests pass with the patch
• Full `./pkg/model/provider/anthropic/...` suite passes
• Full `./pkg/runtime/...` suite passes
• Full `./pkg/model/...` suite passes
• `gofmt` clean, `go vet` clean

Follow-up

`pkg/model/provider/bedrock/adapter.go` carries the same single-`currentToolID` pattern and is likely vulnerable to the same bug on parallel Bedrock Converse tool calls. Not patched here to keep the diff focused — happy to file a follow-up PR if useful.

🤖 Generated with Claude Code

[docker-agent]

Assessment: 🟢 APPROVE

The fix is correct and well-targeted. Replacing the single toolID string field with a toolIDByBlock map[int64]string keyed by content-block index is the right solution for routing InputJSONDelta events across parallel tool_use blocks. Both streamAdapter and betaStreamAdapter are patched consistently, and the new regression tests faithfully reproduce the interleaved-delta scenario described in the PR.

Verification summary

Finding	Severity	Verdict
Silent empty-string ToolCall ID on missing block index	Medium	✅ DISMISSED — Anthropic API guarantees content_block_start before any delta; retry path restarts from the beginning so indices are re-populated before deltas arrive
toolIDByBlock not cleared on stream retry	Medium	✅ DISMISSED — retry replaces the stream from the beginning; all content_block_start events are re-delivered before any deltas, overwriting any stale entries
Redundant nil-guard inside Recv()	Low	Informational note (see inline comment)

One low-severity observation is left as an inline comment for awareness. No blockers found.